[Bug 1309525] Bump up Elixir version

[Red Hat Bugzilla]

https://bugzilla.redhat.com/show_bug.cgi?id=1309525

--- Comment #3 from Martin Langhoff  ---
More than happy to lend a hand. Will need help on a proper Erlang 18 bootstrap.
I made an initial attempt but there's circular deps, and I don't know what
tooling and process was used in the v17->v18 process.

Without some tooling support, it's a pita to run bootstraps, I have jury-rigged
my mock roots to access an "in progress" repo but its rather awkward.

I am super excited as well with Erlang 18 on F24 and I am working with some
very interesting customer projects based on Elixir. This stack has wings.